As per GERMAN MACHINEGUNS by Daniel D. Musgrave on page 326, "Several variations of the 'Ruckstossduse', or 'recoil nozzle,' exist. They differ in the diameter of the hole through which the bullet and the propellant gases pass. The diameter was 11.5mm on the World War II guns; for the same guns this was increased to 14mm after the bolt catch came into use; after the change to 7.62mm NATO ammunition the diameter became 11mm. Later, the recoil booster and the recoil nozzle were made together, I one part."